For women with a regular 28-day menstrual cycle, this week, ending tomorrow, is prime baby-making time, but there are several factors that determine a baby's birthday. Experts warn that conception on Friday doesn't definitively ensure an 11/11/11 baby.
"There's absolutely no guarantee," Rhonda Gannon, a nurse at Boston IVF in Waltham, Mass., told AOL News.
She acknowledges that for women whose last menstrual period was Feb. 4, tomorrow is most likely an ovulation day, making it the perfect time to conceive for a baby due in mid-November.
The calculations for an 11/11/11 baby also presume a 40-week gestation period, which, most parents and obstetricians can tell you, isn't always the case.
Dr. Joe Hill, an obstetrician/gynecologist and fertility expert with the New England Fertility Centers in Portsmouth, N.H., says due dates are an estimate of the baby's arrival, not an absolute.
"Most times, it's a day or two on either side of the due date," Hill told AOL News. "Only the baby knows for sure."
Gannon says her first son was born five weeks early, and her second son arrived two weeks before his due date.
For those who have their hearts set on a specific birthdate, the odds aren't bad. Those under 35, Hill said, have a 1 in 5 chance. Mama wannabes over 35 have a 10 percent chance.
While Hill says he has had patients ask when to conceive to have a child with a certain astrological sign, he hasn't had patients ask him about conceiving a baby to be born on a specific date, such as 11/11.
"Most people just want to have a healthy baby," he said. Which, whether he or she is born on Nov. 11 or Nov. 12, is really all that matters.